Alkaline cells are well known in the art and generally employ a zinc anode, manganese dioxide as the cathode with an aqueous solution of potassium hydroxide for the electrolyte. These cells are readily available commercially for industrial and home applications. Recently a new type of alkaline cell was disclosed by Cegasa International, a Spanish company. This cell, referred to as an air-assisted cell, employs zinc as the anode and manganese dioxide as the cathode with an aqueous solution of potassium hydroxide as the electrolyte. This cell is designed so that the positive electrode containing the manganese dioxide (MnO.sub.2) is supported about its periphery and along its full length in the cell by a perforated ribbed air distribution grid. The bottom or negative end of the cell has an insulating support which allows air to enter the cell and pass up along the outside of the supported positive electrode. When the cell is initially put into a circuit, the electrochemical reaction depends primarily upon the presence of the manganese dioxide cathode. As the reaction progresses, and the manganese dioxide cathode is electrochemically reduced, air within the cell reoxidizes and recharges the manganese dioxide. Thus an air-assisted cell is designed to use oxygen in the air to "recharge" manganese dioxide in the cathode. This "recharging" of the manganese dioxide means that the fixed quantity of manganese dioxide in the cathode can be discharged and then recharged numerous times. In contrast, the cathode's ampere hour output in a standard alkaline battery is limited by the quantity of manganese dioxide incorporated in the cell when the cell is manufactured. Therefore, based upon the cathode's ampere hour input, the maximum service obtainable from an air-assisted alkaline battery is greater than the maximum service which can be obtained from a comparably sized standard alkaline battery. The need to get oxygen to the manganese dioxide in an air-assisted alkaline cell means that a portion of the battery, such as the seal, must be designed to allow oxygen to flow through and directly contact the cathode. Contrary to this, the seal in regular alkaline cells is designed to be air tight.
Different types of seals have been used which permit air to pass through the seals while preventing the ingress and/or egress of undesirable materials into and from the cell, respectively. Specifically, the seal must prevent electrolyte from the cell from passing through the seal. Another desirable feature of the seal is to provide a safety vent which will rupture and release the cell's internal pressure when the internal pressure exceeds a predetermined value. Although a seal can be designed to permit air to enter the cell and prevent electrolyte from escaping from the cell, it is difficult to have the seal with these characteristics also function as a safety vent.
Other galvanic cells, such as alkaline cells, are generally designed to vent when the internal pressure exceeds a predetermined amount. When exposed to an abuse condition, such as being charged to an excessive degree, the cell is designed to vent and allow gas to escape. Under certain abuse conditions, electrolyte entrained in the gas may be forced from the cell. It is preferable to have the electrolyte escape rather than have the cell rupture from internal pressure buildup.
Cell manufacturers have used a number of approaches to resolve the problem of expelling electrolyte during venting. One method of preventing seal rupture due to abuse charging or the like is to insert a diode in the battery's electrical circuit. By eliminating the possibility of charging the cells, internal gas is not generated and the seal never ruptures. Another electrically related mechanism is a belleville shaped "flip switch". This device is triggered by bulging of the closed end of the cell's cylindrical container which causes the belleville member to invert and thereby break electrical contact. Another method involves the use of adsorbents or electrolyte thickeners. The adsorbent materials are usually located outside the seal area and beneath the cell's cover or jacket. As electrolyte escapes from a ruptured seal, the liquid is adsorbed. Spew thickeners are mixed with the electrolyte and therefore are contained within the cell. The objective of the thickener is to slow down and/or adsorb any leakage that may occur. The disadvantage of using either an adsorbent or a thickener is that both materials tie up space that otherwise could be used for active materials of the cell. A third procedure is to use an outer container and end covers as an electrolyte containment system to provide space to contain the electrolyte that may escape.
It is an object of the present invention to provide a seal for galvanic cells that will burst when the internal pressure exceeds a predetermined level.
It is another object of the present invention to provide a multilayer film for a seal assembly for galvanic cells that will burst or blowout when the internal pressure exceeds a predetermined level and wherein the pressure required to blowout the multilayer film is no greater than 20% higher than the pressure to blowout a single layer of the same material having one half the thickness of the multilayer film.
It is another object of the present invention to provide a seal and safety vent assembly for galvanic cells, such as air-assisted cells and alkaline cells, that is easy to make and cost effective to produce.
The above and further objects will become apparent upon consideration of the following description and drawings thereof.